Page 207 - Vitamin D and Cancer

194 F.S.G. Cheung and J.K.V. Reichardt

UVR
UVR
Skin
7-DHC Previtamin D 3 Vitamin D 3

Dietary Vitamin D and D 3
2

Kidney

25(OH)D 3
Liver
Extra-renal tissues
a

b
1,25(OH) D 1,25(OH) D
2 3
2 3
Autocrine or paracrine
Gene transcription regulation of cell growth
for mineral and
bone homeostasis

Fig. 9.1 Vitamin D synthesis in the skin and its actions. Ultraviolet light aids in the conversion
of 7-DHC to previtamin D which thermically isomerizes to vitamin D . Both synthesized and
3
3
ingested vitamin D are hydroxylated in the liver to form 25(OH)D and the kidneys (a) or extra
3
renal tissues (b) to form 1,25(OH) D which acts on target cells to elicit a biological response
2 3
presence of VDR in various cancer cell lines [51]. More importantly, growth inhibitory
effects of 1,25(OH) D have been demonstrated in breast cancer [28], prostate [106,
2
3
121], colon [31, 141] and melanoma cell lines in culture [29]. There is also accu-
mulating evidence on 1,25(OH) D having anti-proliferative, pro-differentiation
3
2
and photoprotective properties in keratinocytes which makes it potentially very
attractive as an anti-cancer agent [38].
Therefore, UVR has a dual effect on skin cancer development and vitamin D
synthesis that is important in maintaining the health of the body as well as prevent-
ing cancer development. Considering that solar dependant vitamin D synthesis
contributes to 90% of the body’s vitamin D requirement [133], when determining
vitamin D recommendation levels, we face the dilemma in seeking a fine balance
between the amount of sun exposure to produce sufficient vitamin D while avoiding
excessive exposure that can increase the risk of skin cancer development.